Transmission line windows having high voltage breakdown characteristic



April 12, 1955 M CLARK, JR 2,706,275

TRANSMISSION LINE WINDOWS HAVING HIGH VOLTAGE BREAKDOWN CHARACTERISTIC Filed Jan. 21, 1946 TO U .4 I /7? SOURCE /Z://// /A M l \32 I l I ATTORNEY United States Patent 1 2,706,275 TRANSMISSION LINE WINDOWS HAVING HIGH VOLTAGE BREAKDOWN CHARACTERISTIC Melville Clark, Jr., Syracuse, N. Y., assigior, by mesne assignments, to the United States of America as represented by the Secretary of the Navy Application January 21, 1946, Serial No. 642,472 4 Claims. (Cl. 33334) This invention relates to seals and more particularly to a pressurized seal for waveguides or coaxial lines.

Heretofore waveguide components, and coaxial line components when operated at high power levels at ordinary air pressure or at ordinary power levels at reduced pressures had a tendency to arc over. It has been the practice to seal the transmission lines with a dielectricmember so that the line could be pressurized. These seals have introduced the disadvantage of disturbing the flow of radio frequency energy in the line and increasing the possibility of breakdown as a result of arc-over across the surface of the seal.

The principal object of this invention is to provide a pressurized seal for a waveguide or a coaxial line which will not arc-over at high power levels and/or reduced pressures.

Another object of this invention is to provide a window or seal so placed in a waveguide or coaxial line that the breakdown strength of the component is a-maximum.

A further object of this invention is to provide dielectric feet for the window or seal thus increasing the breakdown path length.

These and other objects will be apparent from the following specification when taken with the accompanying drawing in which Fig. 1 is a longitudinal section view of a section of wave guide including the inclined window and the associated inductive irises; and

Fig. 2 is a detailed view of a window in a coaxial line.

Referring now to Fig. 1 of the drawing, there is shown a waveguide 14, for transferring power from a source to a load in the form of electromagnetic waves. The particular structure of the waveguide 14, shown in the longitudinal cross-section is unimportant insofar as the principles of the present invention are concerned, and may thus be of rectangular or circular cross-section. The waveguide 14, is sealed by a low loss dielectric window 12, placed at an angle to the longitudinal axis of the guide as shown. The dielectric member 12 is so cut as to fit securely against the inner walls of the transmission line 14.

Dielectric feet 16 are provided at the contacting edges of window 12 and waveguide 14, which feet, as illustrated extend over the surfaces of the window and the waveguide. It is preferred that the window 12 be situated at a point in the waveguide which is at a voltage minimpm in the standing wave pattern ordinarily obtained in transmission lines of the type described.

Since arc-over of a window 12 always occurs over the surface thereof, and since the power' level is generally not limited by the internal breakdown strength of the dielectric but rather by the surface breakdown strength, the maximum power level is increased by increasing the breakdown path length or by decreasing the electric gradient at the surface of the dielectric. The path length is increased in this invention by two methods; first, by sloping the dielectric window 12 as shown in the drawing to increase its overall length in the direction of possible breakdown and secondly; by putting dielectric feet 16 on window 12. The latter may be accomplished simply by painting the edges of guide 14 and window 12 with some high breakdown dielectric paint or by making the feet 16 of similar material as window 12, as shown in the drawing or, as integral extensions of the window itself. It is evident that these methods may be used singly or together. Also, the gradient at the surface of the dielectric is decreased by sloping the dielectric window 12.

Referring again to Fig. 1 there is shown a section of rectangular wave guide 14 including an inclined window 12. The dielectric feet are indicated by reference character 16. Inductive irises 22 and 24 are placed on either side of the window 20. These irises are positioned a predetermined distance apart so that a standing wave 2,706,275 Patented Apr. 12, 1955 voltage pattern exists therebetween. Window 20 is positioned such that its center is located at a low voltage point in this pattern.

The discontinuity introduced by iris 24 is balanced by iris 22 so that no standing waves are introduced in the wave guide 14 between iris 22 and the source of energy due to the introduction of these irises into the wave guide assembly.

In Fig. 2, reference characters 28 and 30 designate the inner and outer conductors of a coaxial transmission line. I n such a line the window preferably has a conical shape as shown at 32. The window is provided with an opening 34 at its apex to receive the inner conductor 28. In this figure dielectric feet 36 are shown formed integrally with window 32 to increase the breakdown path length as ele xplained in detail in connection with the description of The principles described for increasing the surface area of a coaxial line or waveguide window and, consequently, increasing its breakdown voltage, provide a simplified and effective pressurizing and weatherizing seal for the While there is shown an embodiment of this invention, it will be understood that many modifications may be made without departing from the spirit thereof, and it is contemplated by the appended claims to cover any such modifications as fall within the true spirit and scope of this invention.

What is claimed is:

1. In combination, a guided wave transmission line and a dielectric window sealed at its edges to said transmission line, the surfaces of said window being disposed at an oblique angle to the direction of the electric field within the transmission line, a pair of irises disposed within said wave guide transmission line and positioned to cause standing waves to exist therebetween, said dielectric window being positioned such that its center is at a minimum voltage point of said standing waves.

2. In combination with a rectangular wave guide, a pressure seal for said wave guide adaptable for use at high ratios of power level to pressure within said wave guide, said pressure seal comprising a substantially flat dielectric sheet disposed within said wave guide at an oblique angle to the broader walls of said wave guide, said dielectric sheet being sealed to the walls of said wave guide at the edges of said sheet, and a plurality of dielectric feet overlying a portion of said dielectric sheet in the region of the edges of said sheet in contact with said broader walls, said dielectric feet further extending axially of said wave guide in contact with said broader wall, first and second irises disposed on opposite sides of said sheet to produce a standing wave having a voltage minimum at the center of said dielectric sheet.

3. In combination with a rectangular wave guide, a pressure seal for said wave guide adaptable for use at high ratios of power level to pressure within said wave guide, said pressure seal comprising a substantially flat dielectric sheet disposed within said wave guide, said dielectric sheet being sealed to the walls of said wave guide, first and second irises disposed within said wave guides, said irises being positioned to cause standing waves to exist therebetween, said dielectric sheet being positioned such that its center is located at a minimum voltage point in said standing wave.

4. A pressure seal as in claim 3, said seal further comprising a plurality of dielectric feet overlying a portion of said dielectric sheet in the region of the edges of said sheet in contact with said broader walls of said wave guide, said dielectric feet extending axially of said wave guide in contact with said broader walls.

References Cited in the file of this patent UNITED STATES PATENTS Re. 20,244 Peterson Jan. 12, 1937 1,693,02l Cameron Nov. 27, 1928 2,122,893 Roosenstine July 5, 1938 2,207,522 Smith et al July 9, 1940 2,376,725 Richardson May 22, 1945 2,423,492 Fairbank July 8, 1947 2,427,098 Keizer Sept. 9, 1947 2,430,130 Linder NOV. 4, 1947 2,504,494 Bull Apr. 18, 1950 

